1. Field of the Invention
The present invention relates generally to power converters and ballast controls with power factor correction (PFC), and relates more specifically to a single-stage power converter or ballast control that incorporates a PFC circuit.
2. Description of Related Art
Electronic ballasts and power converters that include a power factor correction (PFC) circuit are well known in the relevant industries. Typically, any type of general power converter, which includes electronic ballasts, have connected to their input a PFC circuit to preferably correct the input power factor to unity. It is desirable, and often required according to regulatory demands, that loads connected to power lines appear as purely resistive loads without any connective impedances. That is, the alternating voltage and current supplied by the line input are maintained to be in phase with each other so that the connected load appears purely resistive. When the input voltage and current are in phase, the power factor approaches unity, thereby providing a load that appears to be purely resistive on the input line, without any apparent influence from capacitance or inductance that would otherwise occur if the voltage and current are out of phase with each other.
To achieve a unity power factor, a power factor correction circuit is typically connected to the power line input. The PFC circuit also generally produces a regulated DC bus voltage that is supplied to an inverter for use in power conversion applications. A typical power converter application is an electronic ballast for use with a fluorescent lamp. Often, an electronic ballast consists of a power inverter fed by a DC bus voltage, with the inverter being controlled to provide fluorescent lamp pre-heating, ignition and normal supply power during normal running conditions. A simple block diagram of such an application is provided in FIG. 1. The electronic ballast illustrated in FIG. 1 includes a half-bridge resonant output stage for powering the lamp. The PFC circuit connected to the power line input is typically realized as a boost-type converter that uses a high voltage switch, an inductor, a diode, a high voltage DC bus capacitor and a PFC control circuit. The electronic ballast output stage is typically realized with a half-bridge driven resonant load that uses two high voltage switches, a resonant inductor, a resonant capacitor, a DC-blocking capacitor and a ballast control circuit. A simplified circuit diagram of a conventional electronic ballast circuit is shown in FIG. 2.
The conventional half-bridge electronic ballast output stage configuration shown in FIG. 2 includes a DC bus capacitor Cbus that is connected across the switching half-bridge. As can be seen in the circuit diagram in FIG. 2, high side half-bridge switch M1 and DC bus capacitor Cbus are connected together at a single node. When the conventional electronic ballast of FIG. 2 is switched on, input power is first used to charge DC bus capacitor Cbus, which then supplies power to the half-bridge resonant output stage while the electronic ballast operates. The PFC circuit composed of inductor Lpfc, switch Mpfc and diode Dpfc operate during start-up to charge bus capacitor Cbus. In this conventional circuit typology, power typically flows in a single direction through the load, and bus capacitor Cbus supplies power through the entire cycle of power transfer to the load. Accordingly, bus capacitor Cbus must be rated to withstand peak power transfer, and switches Mpfc, M1 and M2 must also be rated to withstand high peak bus voltages.
It would be desirable to reduce the ratings needed to realize a power converter circuit with an input PFC circuit, and simplify the circuit at the same time.